1. Field of the Invention
The present invention relates to an automotive electric power steering controller for detecting whether a relay has melted and welded at the start of operation.
2. Description of the Related Art
FIG. 9 shows the structure of a known electric power steering controller disclosed, for example, in Japanese Patent Laid-Open No. 64289/1993. This controller has an electric motor 1 for delivering an assisting torque to the steering wheel (not shown) of a vehicle and a battery 2 for supplying motor current IM to the motor 1, for driving it.
A capacitor 3 having a large capacity of about 1000 xcexcF to 3600 xcexcF absorbs the ripple component of the motor current IM. A shunt resistor 4 is used to detect the motor current IM. A bridge circuit 5 is made up of plural semiconductor switching elements Q1-Q4 such as FETs (Field Effect Transistors) for switching the motor current IM according to the magnitude and the direction of the assisting torque. A relay 6 passes or cuts off the motor current IM according to the need.
A driver circuit 7 switches semiconductor switching elements Q1-Q4 and drives the motor 1 via the bridge circuit 5 and actuates the relay 6. A motor current detection means 8 detects the motor current IM from both ends of the shunt resistor 4. The driver circuit 7 and the motor current detection means 8 form an interface circuit for a microcomputer (described later). A torque sensor 9 detects the steering torque T applied to the steering wheel. A vehicle speed sensor 10 detects the vehicle speed V of the vehicle.
The aforementioned microcomputer (CPU) is indicated by numeral 12 and calculates the assisting torque, based on the steering torque T and the vehicle speed V. The microcomputer 12 feeds back the motor current IM and creates a driver signal corresponding to the assisting torque. This microcomputer 12 applies driving signals to the driver circuit 7. One of the driving signals consists of an instruction D0 indicating the direction of rotation for controlling the bridge circuit 5. The other driving signal consists of an amount of controlling current I0.
The microcomputer 12 comprises a motor current determining means 13, a subtractor means 14, and a PID arithmetic means 15. The motor current-determining means 13 creates the instruction D0 indicating the direction of rotation of the motor 1 and an instruction Im indicating an amount of motor current corresponding to the assisting torque. The subtractor means 14 calculates the current deviation xcex94I of the motor current IM from the instructed motor current Im. The PID arithmetic means 15 calculates amounts of correction of proportional (P) term, integral (I) term, and derivative (D) term from the current deviation xcex94I and creates an amount of controlling current I0 corresponding to a pulse width modulation (PWM) duty ratio.
The microcomputer 12 further includes an A/D converter, a PWM timer circuit, and other components (none of which are shown) and has a self-diagnosing function. When the system is started, the microcomputer judges whether a contact of the relay 6 has melted and welded. Also, the microcomputer makes a fault diagnosis on the system. If no fault is found, the microcomputer actuates the relay 6 and supplies electric power to the bridge circuit 5. During operation of the system, the microcomputer constantly makes a self-diagnosis to judge whether the system is operating normally. If a trouble occurs, the microcomputer causes the driver circuit 7 to open the relay 6, thus cutting off the motor current IM.
The operation of this electric power steering system is described by referring to FIG. 9. The microcomputer 12 accepts the steering torque T and the vehicle speed V from the torque sensor 9 and the vehicle speed sensor 10, respectively. The motor current IM is fed back to the microcomputer 12 from the shunt resistor 4. The microcomputer 12 creates the instruction D0 for the direction of rotation of the power steering and the amount of controlling current I0 corresponding to the amount of the assisting torque, and sends these to the driver circuit 7.
Under steady-state operating conditions, the driver circuit 7 closes the relay 6 that is normally open. When the instruction D0 for the direction of rotation and the amount of controlling current I0 are input, the microcomputer creates a PWM driving signal and sends it to the semiconductor switching elements Q1-Q4 of the bridge circuit 5.
Thus, the motor current IM is supplied from the battery 2 to the motor 1 via the relay 6, the shunt resistor 4, and the bridge circuit 5.
The motor current IM is detected via the shunt resistor 4 and via the motor current detection means 8 and fed back to the subtractor means 14 in the microcomputer 12 so that the motor current IM agrees with the instructed motor current Im.
As a result, the motor 1 is driven by the motor current IM and produces a desired amount of assisting torque in a desired direction. The motor current IM contains a ripple component due to switching operation of the PWM drive of the bridge circuit 5. However, the current is smoothed by the large-capacity capacitor 3.
Generally, in this kind of electric power steering system, detection is made to see whether a contact of the relay has melted and welded before closure of the relay when the steering system is started. A known method of detecting whether the relay contact has melted and welded uses a voltage Vrc at the relay contact to which a load is connected. If the contact of the relay 6 has melted and welded, the relay contact voltage Vrc becomes equal to the battery voltage via the relay 6. Therefore, the presence or absence of the melting of the relay can be detected from the magnitude of the relay contact voltage Vrc. However, when the relay is open, the relay contact voltage Vrc becomes equal to the charging voltage for the smoothing capacitor 3. Consequently, where the electric power steering controller is restarted immediately after turned off, for example, the detection is performed when the smoothing capacitor 3 is not yet fully discharged. That is, the detection whether melting has occurred is done while the relay contact voltage Vrc is high. Hence, fault of the relay 6, i.e., melting and welding, may be misdiagnosed.
To avoid this, in the prior art electric power steering system described above, when melting and welding of the relay contact are detected, the microcomputer must wait until the smoothing capacitor 3 is fully discharged. This prolongs the starting time of the electric power steering system. A readily conceivable method of shortening the discharge time of the smoothing capacitor 3 is to insert a resistor in parallel with the smoothing capacitor 3 before discharging is performed. In this method, however, the electric current consumed when the electric power steering controller is in operation increases. Furthermore, a discharging resistor having a large power loss is necessary.
The present invention has been made to solve the foregoing problems. It is an object of the present invention to provide an electric power steering controller in which a smoothing capacitor is discharged with low power consumption to thereby shorten the discharge time, thus shortening the starting time.
An electric power steering controller in accordance with the present invention is adapted to give an assisting torque from an electric motor to a steering member for a vehicle, said electric power steering controller comprising:
a relay containing a first contact at a side of the electric motor and a second contact at a side of a DC power supply and switching a circuit between the electric motor and the DC power supply;
a smoothing capacitor connected with said first contact;
a voltage detecting means for detecting a voltage at said first contact; and
a discharging circuit having a switching means disposed in a discharging circuit for discharging said smoothing capacitor, said switching means being turned on before the relay is driven so as to turn on said first and second contacts.
According to this invention, the smoothing capacitor is discharged in a shorter time. The time taken to detect melting and welding of the relay contact is shortened. Therefore, the starting time of the controller is reduced.
In one feature of the electric power steering controller if said smoothing capacitor is charged to above a given voltage, the smoothing capacitor is discharged to below the given voltage by said switching means.
In this feature, the electric power steering controller can be started in a shorter time by performing discharging only when necessary.
In another feature of the electric power steering controller, said voltage detecting means detects the voltage at said first contact after said smoothing capacitor is discharged by said switching means of the discharging circuit.
In this feature, melting and welding of the contact can be detected with certainty.
In a further feature, the electric power steering controller further comprises a charging circuit for charging said smoothing capacitor, and wherein said charging circuit is so controlled as to charge said smoothing capacitor after said voltage detecting means detects the voltage at said first contact and before said relay is driven to turn on said first and second contacts.
In this feature, the provision of the means for charging the smoothing capacitor can reduce the current through the relay contact. In consequence, melting and welding of the relay contact can be prevented.
In a still other feature of the electric power steering controller, said discharging circuit has a resistor connected between said switching means and said smoothing capacitor, and wherein said resistor is also used as a charging resistor in said charging circuit.
In this feature, because the discharging circuit and the charging circuit can share the resistor in this structure, the number of components in the circuit can be reduced.
In a yet other feature of the electric power steering controller, if said voltage detecting means detects a voltage in excess of a given voltage after said smoothing capacitor is discharged by said switching means, said discharging circuit is so controlled that said smoothing capacitor is discharged again by said switching means after a lapse of a certain wait time.
In this feature, melting and welding of the contact can be detected more reliably.
In an additional feature of the electric power steering controller, said wait time is controlled so as to vary according to the voltage at said first contact.
In this feature, the electric power steering controller can be started more quickly.
Other objects and features of the invention will appear in the course of the description thereof, which follows.